Change return type of Cancel() to bool.

This should fix the following warning on windows:

..\src\binding.cc(117): warning C4800: 'int' : forcing value to bool 'true' or
'false' (performance warning)

Spotted in issue #165.

src/binding.cc

@@ -301,7 +301,7 @@ protected:
     return rv;
   }
 
-  int Cancel()
+  bool Cancel()
   {
     PGcancel* pgCancel = PQgetCancel(connection_);
     char errbuf[256];